The field of the invention relates generally to preforms for composites, and more specifically to methods and apparatus associated with narrow tows fabricated from wide, unidirectional, large-tow-based preforms that incorporate a large number of fibers.
Fiber-reinforced composite structures are sometimes produced utilizing preforms. A preform is generally prepared by building up successive layers of carbon-fiber fabric. Strands of continuous filaments, which are sometimes referred to as tows, of carbon fiber may be woven into a two- or three-dimensional fabric, braided into a two- or three-dimensional fabric form, or combined in other fashions. The fabric produced in this fashion may then be assembled into a near-net shape and then infused with resin and cured in situ in a mold under heat and pressure to form a part.
It is desired to utilize low-cost preforms in the manufacture of composite structures. However, these generally entail the use of fiber tows having a large number of fibers therein. When these tows that incorporate a large number of fibers, or large fiber tows, are used, the resulting fabric quality is often below the fabric quality of a fabric made using similar forms and fiber tows that are fabricated with a lesser number of carbon fibers therein (smaller fiber tows), even though these fabrics require a corresponding increase in the number of these smaller fiber tows used in the construction of the fabric.
This problem is compounded when braids or woven fabrics are being fabricated. The use of large fiber tows in braids leads to large interstitial spaces between tows that become resin-rich pockets after infusion with resin. These resin-rich pockets are prone to the development of undesired microcracks. The woven fabrics fabricated using large fiber tows often have problems during handling because there is a relatively small number of fiber overlaps between the individual large fiber tows as compared to fabrics that incorporate smaller-fiber tows in the fabrics.
However, utilization of small fiber-based tow fabrics (tows that incorporate fewer fibers) in the fabrication of composite materials is far more expensive than utilization of tows incorporating a large number of fibers. This counterintuitive expense is at least partially based on the capacity of the machines utilized in the fabrication of carbon-fiber tows. Generally, the fabrication of a 24K tow, containing 24,000 carbon fibers, polyacrylonitrile (PAN) tows, also containing 24,000 filaments, are passed through a series of high-temperature ovens in which the PAN is carburized to carbon to form tows with an identical (here 24,000) number of filaments as the PAN precursor. To fabricate a 1K tow, an identical process is used, but PAN tows that contain only 1,000 filaments are used as the precursor. Typically, the carburization apparatus used to convert PAN to carbon can handle a fixed number of tows and the linear rate of conversion (meters/minute, for example) is essentially identical irrespective of the size of the precursor tow (e.g. 1K vs. 24K). For example, twenty-four 1K PAN precursor tows would be needed to yield an equivalent amount of carbon per hour that could be fabricated with one 24K PAN tow. In this very simple example, the fabrication process is running at 1/24 of the efficiency when fabricating 1K tows, which is one reason that 1K tows are generally much more expensive than 24K tows.